Buffer-like Action in Water Pool of Aerosol OT Reverse Micelles

Abstract

The local pH in the water pool of Aerosol OT reverse micelles in n-heptane was estimated from the excitation spectra of the pH-sensitive fluorescence probe, pyranine, solubilized into the micellar core as a function of pH of the aqueous solutions to be solubilized. This probe shows two distinct excitation bands corresponding to the neutral (nonionized) and basic (ionized) forms. The intensity ratios of these bands were in advance plotted as a function of the pH of the aqueous solutions to make a calibration curve for local pH estimation. The probe-containing aqueous solutions ranging from pH 1 to 12, adjusted using NaOH/HCl or buffers, were solubilized into the reverse micelles at a constant water content (RW = [H2O]/[AOT] = 20). When NaOH/HCl was used for pH adjustment, even if an acidic or alkaline aqueous solution was solubilized into the micelles, the solubilized probes reported unexpectedly an almost constant intensity ratio over a wide pH range. This result suggests that the water pool of AOT reverse micelles has buffer-like action. A similar effect was also observed even when various pH values of buffer solutions were solubilized. The use of fluorescein, instead of pyranine, led to the same conclusion. A model experiment suggested that such buffer-like action is related to a considerably high concentration of the AOT sulfonate groups localizing on the water/oil interface. The mechanism can be explained in terms of the restricted electrolytic dissociation of the AOT sulfonate groups in the reverse micellar aggregation state.